The mammalian cell line Chinese Hamster ovary (CHO) has emerged as an outstanding model mammalian system. Its chromosome complement is stable and amenable to genetic analyses. Conditional lethal mutants and revertants for aminoacyl-tRNA synthetases can be isolated. This study will isolate and purify leucyl-tRNA synthetase from wild type, mutant, and revertant cells. The particulate nature of the enzymes and role of low molecular weight RNAs and lipids to the enzyme and subribosomal particles will be studies. Methodologies used will be, cell culture, sucrose gradients, chromatography (G.C., ion exchange, RPC-5 for tRNA isoacceptors), centrifugation, and electrophoresis. Little is known about the molecular events occurring in mammalian cells and biochemical studies combined with genetics are essential.